Transmitters typically implement a linear power amplifier to amplify modulated signals that have a time-varying amplitude (magnitude) for transmission. It is desirable for the linear power amplifier to provide good linearity and efficient power conversion. Class B or AB power amplifiers are typically the most suitable amplifiers for obtaining a best efficiency relative to distortion. However, many communications applications require a further reduction in amplifier distortion, which may be obtained by negative feedback. A cartesian loop is a known method for implementing negative feedback around a linear power amplifier. A net phase shift around the cartesian loop must be maintained near 180 degrees at a desired channel frequency in order to insure stable operation. Component variability, time delay in the loop, and other factors can cause the loop phase shift to vary considerably. Therefore, in order to keep the cartesian loop stable in the presence of phase shift variation, methods for measuring and adjusting the loop phase shift have been proposed. However, these earlier methods have required low frequency sine waves as input signals, resulting in somewhat complex phase adjustment computation.
Phase shift compensation in cartesian-loop transmitters has been utilized, but has required at least 40 milliseconds. There is a need for a faster phase shift compensation method for a linear transmitter using negative feedback to allow more time for productive use of a transmitted signal.